1. Field of the Invention
This invention relates to a distributed control system which controls process variables, such as, for example, temperature, pressure, etc. More particularly, the invention relates to such system comprising a control station which controls various operations, and a plurality of remote input/output devices which are distributed separately from the control station to exchange signals with sensors, actuators, etc. More specifically, the invention relates to transmission control of a data bus which connects the control station to the remote input/output devices.
Furthermore, the invention relates to measures which are undertaken to cope with power failure in such a distributed control system where a plurality of independent power supplies are provided.
2. Description of the Prior Art
A typical distributed control system is supplied with a large number of input signals from a process as a controlled system and outputs manipulated variable signals to a large number of control elements. In order to allow the entire controlled system to be monitored and controlled comprehensively, a plurality of remote input/output devices are connected to the control station in accordance with the nature of each particular task assigned thereto.
Between the control station and the remote input/output devices there is provided a shared memory to which read/write access can be made from both the control station side and the remote input/output device side, and cyclic scan transmission is carried out for each remote input/output device through a remote input/output bus so that the contents of a data buffer used by a microprocessor in the control station and input/output data on the remote input/output device side are maintained equivalently.
Such cyclic scan transmission is carried out according to preset transmission conditions, for example, as follows.
(a) The correspondence between the input/output nodes of the remote input/output devices and the respective addresses on the shared memory.
(b) The cyclic scan transmission cycle used for each input/output node.
(c) The type of transmission service which is to be performed for each input/output node, such as, reading of input data, exchange of output data for read-back data, etc.
A typical conventional system of the type just described is arranged such that when a power supply for the system fails, and then recovers from the failure, the microprocessor in the control station will again execute the above described cyclic scan transmission.
As can be readily understood, the convention system has many problems and deficiencies, and leaves room for improvement. For example, the convention system suffers from the problem that when the power supply recovers from a power failure, the normal transmission control, which is carried out by using the remote input/output bus, cannot be started promptly because the microprocessor in the control station has been engaged in various functions to recover from the power failure and a large amount of time is required for the system to recover to a normal process control operation.